Technology of preparing halogenated aromatic compounds by aromatic compounds such as benzene or naphthalene to react with halogen (bromine, chlorine, iodine, etc) has been used in various commercial fields.
Typically, p-dichlorobenzene, which is prepared through the reaction of benzene with chlorine, is used as a raw material for preparing engineering plastic PPS (polyphenylene sulfide). Technology of preparing PPS by allowing p-dichlorobenzene to react with sodium sulfide in an N-methylpyrrolidone solvent is known as the Macallum process, and PPS is currently commercially produced through the Macallum process. However, because it is difficult to obtain a high-molecular-weight polymer only through the Macallum process, a curing process, as a post-process, is carried out to obtain the high-molecular-weight polymer, and PPS obtained through the curing process has a disadvantage in that it becomes brittle due to a crosslinking reaction or the like. Also, metal salts, such as sodium chloride (NaCl), are necessarily produced as reaction byproducts in the polymerization process, and cause serious problems in terms of the economic efficiency of commercial processes and the physical properties of the polymer.
As methods which can fundamentally eliminate the production of metal salts and enable linear polymers to be obtained, U.S. Pat. Nos. 4,746,758 and 4,786,713 and related patents suggest methods of melt-polymerizing p-diiodobenzene with sulfur.
Also, U.S. Pat. Nos. 4,778,938 and 4,746,758 disclose methods of preparing p-diiodobenzene by allowing benzene to react with iodine in the presence of oxygen over a zeolite catalyst. These patents disclose that a conversion rate to a diiodo compound is high, a selectivity to a p-diiodo compound, which is commercially useful, is high, and the oxidation of benzene or naphthalene as a raw material can be minimized.
However, in order to make this iodination technology commercially more useful, it is preferable to further increase the productivity of diiodo compounds and the selectivity to a p-diiodo compound. Also, said patents disclose that carbon deposits are produced due to the combustion of raw material and that the activity of the catalyst is reduced due to the carbon deposits. Furthermore, the carbon deposits thus produced or multi-iodinated high molecular impurities not only deactivate the catalyst, but also remain in the iodinated product, thus causing serious problems in a subsequent purification process.